Cure system for polyisoprene rubber

ABSTRACT

The present invention is based upon the unexpected finding that polyisoprene rubber which is clear and of high purity can be cured utilizing the curative system of this invention to attain an elongation at break of greater than 1200% without compromising the clarity or purity of the rubber. The curative system of this invention is comprised of a carbonyl inhibited platinum catalyst, and a branched crosslinking agent having at least 2 hydrosilyl groups per molecule. The curative system of this invention can be used to cure solid polyisoprene rubber that is used in a wide variety of application and is a particular value in curing polyisoprene rubber used in manufacturing sporting goods, healthcare, and medical products. The present invention more specifically discloses a curable polyisoprene rubber composition comprising: (1) a polyisoprene rubber comprised of repeat units that are derived from isoprene monomer, (2) a carbonyl inhibited platinum catalyst, and (3) a crosslinking agent having at least 2 hydrosilyl groups per molecule.

BACKGROUND OF THE INVENTION

The use of natural rubber in the medical and health care industries canexpose atopic individuals to latex allergens. It is believed that asmuch as 6% of the general population, and as high as 12% of thoseworking in the medical profession, are latex sensitive and displayallergic reactions when exposed to proteins found in natural rubber (seeM. McNulty, Rubber & Plastics News Jun. 25, 2001, page. 5). The symptomsof latex allergy range from mild contact dermatitis to life-threateninganaphylaxis, which includes a rapid drop in blood pressure anddifficulty breathing. Even though a heightened awareness of latexallergy now exists, the number of people becoming sensitized to naturalrubber is increasing as more professions require the use of latex glovesto avoid exposure to infectious agents. It is therefore not surprising,that with over 40,000 consumer products containing natural rubber (seeInformation from Allergy Advisor-Zing Solutions,http://allergyadvisor.com) an alternative protein free material isdesired in many applications.

Although techniques exist for enzymatic deproteinization of naturalrubber (see S. Kawahara, T. Kakubo, N. Nishiyama, Y. Tanaka, Y. Isono,J. T. Sakdapipanich, J. Appl. Polym. Sci. 78, 1510 (2000) and A. H. Eng,S. Kawahara, Y. J. Tanaka, Nat. Rubb. Res. 8, 109 (1993), as well asmanufacturing practices that lower the total allergens present in latexgoods, currently, the most effective way to provide protein freeproducts is to use petrochemical derived synthetic rubbers. In fact, areport from the Johns Hopkins University School of Medicine recommendedthat the Food and Drug Administration mandate a switch from usingstoppers made with natural rubber to using all synthetic rubber medicinestoppers (again see M. McNulty, Rubber & Plastics News Jun. 25, 2001).The synthetic rubber most closely related to natural rubber (NR), ishigh cis-polyisoprene. Typically, this material is prepared through theuse of either stereospecific titanium catalysts or with alkyl-lithiuminitiators. Both of these systems are effective at providing proteinfree synthetic polyisoprene, however, the two polymers differ greatlywith respect to their micro- and macrostructure. Polymerization ofisoprene with titanium tetrachloride activated with a trialkylaluminumco-catalyst results in a material with upwards of 98% 1,4-cis content(see W. Cooper, in W. M. Saltman, ed., The Stereo Rubbers, John Wiley &Sons, New York, 1977, page 48).

Polyisoprene produced commercially with alkyl lithium initiatorsgenerally does not have a cis content higher than 92%. Differences inmicrostructure, as well as macrostructure, allows the titaniumpolyisoprene (Ti—PI), but not the lithium polyisoprene (Li—PI), todisplay the unique advantage of strain-induced crystallization. It isthe property of rapid crystallization that allows NR and Ti—PI to havehigh tensile strength and modulus even without the use of reinforcingfillers, a condition often found in gum stocks commonly used in theproduction of medical goods (see A. R. Bean, Jr., et. al., in H. F.Mark, N. G. Gaylord, N. M. Bikales, ed., Encyclopedia of Polymer Scienceand Technology, John Wiley & Sons, New York, Vol. 7, 1967, page. 823).Although Li—PI lacks the microstructural regularity of Ti—PI, it doeshave the advantage of being gel free with a narrow molecular weightdistribution and linear macrostructure. These attributes allow Li—PI todisplay lower hysteretic properties at a lower cross-link density thanNR or Ti—PI.

The use of Ti—PI and Li—PI in the medical and health care industries hasgained acceptance, yet there is still need for a number of improvements.For example, the consistency of Ti—PI is very dependent on the aluminumto titanium ratio that is used during catalyst preparation (see W.Cooper, in W. M. Saltman, ed., The Stereo Rubbers, John Wiley & Sons,New York, 1977, page 48). If the ratio drops below unity the titanium isnot sufficiently reduced, causing the formation of gel in the polymer.On the other hand, if the catalyst is over-reduced, oligomers will beproduced giving the material a strong odor. Both of these unwanted sidereactions must be carefully controlled. Current titanium systems alsosuffer from inferior activity and high levels of titanium must be usedresulting in elevated levels of catalyst residues and terminating agentsin the finished polymer. An overall reduction in foreign substancesremaining in synthetic polyisoprene is of paramount importance when theproduction of a clean high performance material is desired.

Li—PI is considered a clean polymer due to the use of low levels ofinitiator during production and lack of extractables. However, thedependence of cis content on lithium concentration leads to a polymerwith very high molecular weight (see H. L. Hsieh, R. P. Quirk, AnionicPolymerization Principles and Practical Applications, Marcel Dekker,Inc., New York, 1996, p. 201). The high molecular weight, coupled with anarrow molecular weight distribution, makes processing this materialdifficult. Commonly a low molecular weight fraction is added tocommercial material to act as a processing aid.

The use of neodymium catalyzed polyisoprene (Nd—PI), as a source ofprotein free synthetic natural rubber offers the combined advantages ofboth Ti—PI and Li—PI without their respective disadvantages. Nd—PI witha cis 1,4-content as high as 98%, gel and oligomer free, linear with amoderate molecular weight distribution, easy to process, and low involatile and extractable residues is ideally suited for many clean highperformance applications (see U.S. patent application Ser. No.10/273,918).

The technique of combining a neodymium salt, an aluminum alkyl, a halidesource, and a diene to attain an improved result is the subject of U.S.patent application Ser. No. 10/368,660, filed on Feb. 18, 2003. As theprior art describes, most any conjugated diene monomer can be used inthe preforming step, and that each diene can be treated in the same way.For example, prior teachings imply that the contact time between theconjugated diene and the neodymium/aluminum co-catalyst treatment stepis not crucial and that aging should occur after the halide source hasbeen added. However, U.S. patent application Ser. No. 10/368,660explains that a minimum contact time does indeed exist for differentconjugated dienes when the preparation of a completely soluble catalystis desired. It is also crucial that this contact time occurs prior tothe introduction of a halide source in order to ensure completelysoluble catalyst solutions. For example, formation of a homogeneouscatalyst solution is achieved when isoprene is used in the preform onlyif the isoprene/neodymium/aluminum alkyl solution is allowed to age foran extended amount of time prior to aluminium-chloride addition. If thefirst step is not allowed to proceed long enough, a precipitate isformed upon addition of aluminum-chloride. When butadiene is used in thepreforming reaction this first aging period is still crucial, yet,significantly less time is needed to ensure a homogenous catalyst.

The technological advantage of a completely soluble preformed catalysthas previously been appreciated. As U.S. Pat. No. 4,461,883 teaches, aheterogeneous system is a disadvantage in an industrial setting.Likewise, U.S. Pat. No. 6,136,931 states that the use of heterogeneouscatalyst systems containing suspended particles usually produces gel.This patent also states a heterogeneous system, compared to a homogenousone, is more difficult to control the exact amount of catalyst addedduring the polymerization. Similarly, U.S. patent application Ser. No.10/368,660 indicates that catalyst prepared without the first agingperiod results in a catalyst suspension of a fine precipitate. Thissuspension settles upon standing into two phases. If the resultingsupernate, or top layer, is used to polymerize a conjugated diene,extremely inefficient catalyst activity results. Catalyst activity canbe restored in these systems only after agitation of the by-phasicmixtures. This allows for the introduction of a heterogeneous catalystsuspension to the monomer to be polymerized. However, it is now possibleto ensure consistent and highly active soluble preformed catalystformation by utilizing the appropriate two step aging technique. This isof obvious technological advantage, since there would be no need to usea stirred tank catalyst storage tank or other engineering constraints toensure consistent catalyst suspensions.

The neodymium catalyst system prepared by the technique described inU.S. patent application Ser. No. 10/368,660 can be used in thepolymerization of isoprene monomer into polyisoprene rubber that isclear (transparent) and of high purity. U.S. patent application Ser. No.10/368,660 more specifically discloses a process for the synthesis ofpolyisoprene rubber which comprises polymerizing isoprene monomer in thepresence of a neodymium catalyst system, wherein the neodymium catalystsystem is prepared by (1) reacting a neodymium carboxylate with anorganoaluminum compound in the presence of isoprene for a period ofabout 10 minutes to about 30 minutes to produce neodymium-aluminumcatalyst component, and (2) subsequently reacting the neodymium-aluminumcatalyst component with a dialkyl aluminum chloride for a period of atleast 30 minutes to produce the neodymium catalyst system.

It is, of course, important for high purity polyisoprene rubbers to becured using a technique that will not compromise the purity or clarityof the rubber. By the same token, it is also important for cured rubberarticles made with such polyisoprene rubber to exhibit good mechanicalproperties. For instance, in many applications it is important for thecured polyisoprene rubber to have the highest possible elongation atbreak.

SUMMARY OF THE INVENTION

The present invention is based upon the unexpected finding thatpolyisoprene rubber which is clear and of high purity can be curedutilizing the curative system of this invention to attain an elongationat break of greater than 1200% without compromising the clarity orpurity of the rubber. The curative system of this invention is comprisedof a carbonyl inhibited platinum catalyst, and a crosslinking agenthaving at least 2 hydrosilyl groups per molecule.

The curative system of this invention can be used to cure polyisoprenerubber that is used in a wide variety of applications and is aparticular value in curing polyisoprene rubber used in manufacturingsporting goods, healthcare, and medical products. Some specificapplications include: bandages, drug delivery patches, suture tape,medical devices, pharmaceutical closures, including syringe plungers andsleeves, needle shields, injection sites, vial closures and stoppers,tourniquets, exercise bands, cosmetics, condoms and prophylacticdevices, gloves and glove dip, IV and other bags, tubing, clean roomapplications, baby bottle nipples and teething devices, dental andorthodontic applications, chewing gum and slow release delivery gums,food additives, packaging and delivery systems for food includingbeverage tubing, conveyor belting and rollers, seals and coatings,potable water systems, pet chew toys, adhesive tapes, duct tapes,labels, envelopes, sticky notes, coatings, sealants, including cold sealadhesives for candy wrappers, diaper tabs, elastic thread for garmentsand sporting goods, calendered stock for clothing, rubber bands, rubbergrips for tools and appliances, liquid crystal encapsulation, wire,cable and component insulation, membranes, vacuum applications,computers and communication devices.

The present invention more specifically discloses a curable polyisoprenerubber composition comprising: (1) a polyisoprene rubber comprised ofrepeat units that are derived from isoprene monomer, (2) a carbonylinhibited platinum catalyst, and (3) a crosslinking agent having atleast 2 hydrosilyl groups per molecule.

The subject invention also reveals a process for manufacturing anelastomeric article by compression molding, said process comprising thesteps of: (I) compressing a curable polyisoprene rubber compositionwhich is comprised of (1) a polyisoprene rubber comprised of repeatunits that are derived from isoprene monomer, (2) a carbonyl inhibitedplatinum catalyst, and (3) a crosslinking agent having at least 2hydrosilyl groups per molecule into a mold of the desired shape; (II)curing the curable polyisoprene composition in the mold at a temperaturewhich is within the range of 100° C. to 250° C. to produce theelastomeric article; and (III) removing the elastomeric article from themold.

DETAILED DESCRIPTION OF THE INVENTION

Virtually any polyisoprene rubber having a 3,4-microstructure content(vinyl content) of at least 0.5% can be cured by utilizing the techniqueof this invention. The polyisoprene rubber that can be cured inaccordance with this invention will typically by a polyisoprenehomopolymer. However, the polyisoprene rubber can, optionally, containup to about 40 weight percent of other conjugated diolefin monomers,such as 1,3-butadiene, and/or vinyl aromatic monomers, such as styrene.Such copolymers will typically contain 70 weight percent to 99 weightpercent isoprene monomer and 1 weight percent to 30 weight percent ofthe other copolymerizable monomers.

One representative example of a polyisoprene that can be cured inaccordance with this invention can be synthesized in accordance with thetechnique disclose in U.S. patent application Ser. No. 10/368,660, filedon Feb. 18, 2003. This technique involves polymerizing isoprene monomerin the presence of a neodymium catalyst system, wherein the neodymiumcatalyst system is prepared by (1) reacting a neodymium carboxylate withan organoaluminum compound in the presence of isoprene for a period ofabout 10 minutes to about 30 minutes to produce neodymium-aluminumcatalyst component, and (2) subsequently reacting the neodymium-aluminumcatalyst component with a dialkyl aluminum chloride for a period of atleast 30 minutes to produce the neodymium catalyst system to produce thepolyisoprene rubber. The teachings of U.S. patent application Ser. No.10/368,660 are incorporated herein by reference.

Curable compositions that are suitable for use in molding applicationscan be made by blending the polyisoprene rubber with the hydrosilylationcatalyst and the crosslinking agent. The crosslinking agent willtypically be used at a level which is within the range of 0.5 phr (partsby weight per 100 parts by weight of the polyisoprene rubber) to 10 phr.The crosslinking agent will preferably be present in the curablecomposition at a level which is within the range of 1 phr to 6 phr. Thecrosslinking agent will more preferably be present at a level of 2 phrto 4 phr. The hydrosilylation catalyst will typically be employed at alevel which is within the range of 1 ppmr (parts of metal catalyst byweight per 1,000,000 parts by weight of the polyisoprene rubber) to 200ppmr. The hydrosilylation catalyst will preferably be present in thecurable composition at a level which is within the range of 20 ppmr to100 ppmr. In curable compositions that are used in compression moldingapplications the crosslinking agent will normally be present at a levelwhich is within the range of 2 phr to 4 phr and the hydrosilylationcatalyst will normally be present at a level which is within the rangeof 25 ppmr to 75 ppmr. However, it should be noted that the exact levelof crosslinking agent and catalyst required will depend upon thecharacteristics of the polyisoprene rubber and will normally bedecreased with increasing levels of vinyl microstructure content in thepolyisoprene rubber.

The crosslinking agents that can be used in the practice of thisinvention have at least 2 hydrosilyl groups per molecule. Crosslinkingagents of this type are described in detail in U.S. Pat. No. 6,087,456.The teachings of U.S. Pat. No. 6,087,456 are incorporated herein byreference with respect to teaching the type of crosslinking agent(curing agent) that can be utilized in the practice of this invention.Some preferred branched crosslinking agents are of the structuralformula:

wherein n represents an integer from 1 to about 3, wherein R representsan alkyl group containing from 1 to 4 carbon atoms, a phenyl group, or ahydrosilyl group. The crosslinking agent will typically be atetrakis(dialkyl siloxy) silane or a tris(dialkyl siloxy) alkyl silane.The crosslinking agent will more typically be a branched silane couplingagent such as tetrakis(dimethyl siloxy) silane, tris(dimethyl siloxy)methyl silane, and tris(dimethyl siloxy) phenyl silane.

A wide variety of hydrosilylation catalysts can be used in making thecurable compositions of this invention. Some representative examples ofsuitable hydrosilylation catalysts include chloroplatinic acid,elemental platinum, solid platinum supported on a carrier (such asalumina, silica or carbon black), platinum-vinylsiloxane complexes {forinstance: Pt_(n)(ViMe₂SiOSiMe₂Vi)_(n) and Pt[(MeViSiO)₄]_(m)},platinum-phosphine complexes {for example: Pt(PPh₃)₄ and Pt(PBU₃)₄}, andplatinum-phosphite complexes {for instance: Pt[P(OPh)₃]₄ andPt[P(OBu)₃]₄}, wherein Me represents methyl, Bu represents butyl, Virepresents vinyl and Ph represents phenyl, and n and m representintegers. The platinum-hydrocarbon complex described in thespecification of U.S. Pat. No. 3,159,601 and U.S. Pat. No. 3,159,662,and the platinum-alcoholate catalyst described in the specification ofU.S. Pat. No. 3,220,972 can also be used. The teaching of U.S. Pat. No.3,159,601, U.S. Pat. No. 3,159,662, and U.S. Pat. No. 3,220,972 areincorporated herein by reference.

Hydrosilylation catalysts containing metals other than platinum can alsobe used in the practice of this invention. Some representative examplesof such catalysts include: RhCl(PPh₃)₃, RhCl₃, Rh/Al₂O₃, RuCl₃, IrCl₃,FeCl₃, AlCl₃, PdCl₂.2H₂O, NiCl₂, TiCl₄, and the like. These catalystscan be used alone or in combination. In view of catalytic activity,chloroplatinic acid, platinum-olefin complex, platinum-vinylsiloxanecomplex, and Pt(acac)₂, are preferable.

Inorganic fillers can also be added to the moldable compositions of thisinvention to enhance physical properties. Some representative examplesof inorganic fillers that can be used include calcium carbonate, talc,silica, carbon black and other ordinary inorganic fillers. Silicafillers will typically be added at a level which is within the range of5 phr to 40 phr and will preferably be added at a level of 15 phr to 30phr. Since the curable composition of the present invention is formed bycrosslinking by a hydrosilylation reaction, however, influences on thehydrosilylation reaction should be taken into consideration in usingsuch a filler. For instance, if the filler has a high content ofabsorbed moisture, the moisture will react with the curing agent, whichcan result in foaming during the curing step. When the filler contains acomponent capable of interfering the hydrosilylation reaction, forexample a nitrogen and/or sulfur atom, a reduction in curability orinsufficient curing may result. Some fillers can have an influence onthe storage stability of the curable composition. In using such aninorganic filler, it is important to confirm the influence of theinorganic filler on the curability and/or storage stability beforehand.If visual clarity is being sought it is also, of course, important toavoid fillers that will significantly reduce the clarity of thecomposition, such as carbon black.

One or more of antioxidants, ultraviolet absorbers, pigments,surfactants and other additives can also be incorporated in the curablepolyisoprene rubber compositions of this invention in appropriateamounts. Again, the influence of these agents on the hydrosilylationreaction should also be taken into consideration.

EXAMPLE 1

This experiment demonstrates the unexpected dependence ofhydrosilylation cure of polydiene rubber on branched silane couplingagents. Polyisoprene (400 grams) was milled according to ASTM D 3182 anda smooth sheet was obtained. The silane hydride was added next as aliquid and milled according to ASTM D 3182. After incorporation of thesilane hydride, the noble metal catalyst was added dropwise and milledaccording to ASTM D 3182. When a silica filler is used, it can be addedslowly at this point to the rubber while being milled. After all thecomponents of the compounded rubber were mixed on the mill, the flatmilled sheet was converted into cured tensile sheets by use of a 0.075inch×6 inch×4 inch mold cavity. The tensile sheets were cured at avariety of mold temperatures and cure times. See Table 1 values measuredon an Instron 5500R tensile testing machine were obtained for tearstrength, tensile strength and elongation at break, also RPA Alpha 2000values were obtained during sample curing at 356° F. As is evident fromthe data, branched silanes are unique under these conditions in theirability to undergo productive cross-linking in diene based rubbers. Itshould be noted that the linear di-silanes were ineffective forincreasing the level of elongation. TABLE 1 Hydrosilylationvulcanization Moles Pt Level Elongation Tear Silane Type Si—H (ppm) (%)(ppi) Tensile (psi) Branched tetrakis- 0.043 45 1255 93 467 silaneLinear oligomeric- 0.015 45 not measured under cured silane Lineardi-silane 0.13 45 not measured under cured Linear di-silane 0.35 45 18553 205

EXAMPLE 2

This experiment serves to demonstrate the unexpected increase in curedrubber elongation that is observed when hydrosilylation vulcanization asdescribed in Example 1 is compared to traditional sulfur or peroxidecompounding. For sulfur vulcanization, polyisoprene was initially mixedwith 2 phr (parts by weight per 100 parts by weight of rubber) of zincoxide and 2 phr of stearic acid to make a non-productive compound. Aproductive compound was subsequently prepared by mixing thenon-productive compound with 2 phr of methyl tuads and 0.5 phr ofsulfur. The compounds were then cured and the physical properties of thesamples were evaluated. Likewise, for peroxide based vulcanization, therubber was mixed with 0.4 phr of DI-CUP R peroxide and 2 phr of SartomerSR350 peroxide co-curing agent. The compounds were then cured and thephysical properties of the samples were evaluated. The tensile strength,elongation at break and tear strength of the cured rubber samples isreported in Table 2.

As is evident from the comparison of cure methods in Table 2,hydrosilylation as described in Example 3 leads to an unexpected andsignificant increase in the cured rubber elongation. TABLE 2 Improvedelongation Elongation Tear Tensile Cure Method (%) (ppi) (psi)hydrosilylation 1255 93 467 peroxide 695 85 647 sulfur 715 142 2267

EXAMPLE 3

This experiment serves to demonstrate the effectiveness ofhydrosilylation as practiced in this invention at various cure times andtemperatures. In the procedure used polyisoprene (400 grams) was milledsmooth as in Example 1. The preferred tetrakis-silane hydride (3 phr)was added next followed by 23 ppm of Pt catalyst. After all thecomponents of the compounded rubber were mixed on the mill, the flatmilled sheet was converted into cured tensile sheets by use of a 0.075inch×6 inch×4 inch mold cavity. The tensile sheets were cured at avariety of mold temperatures and cure times. See Table 3 where thevalues determined utilizing the Instron 5500R tensile testing machinewere obtained for tear strength, tensile strength and elongation atbreak. TABLE 3 Hydrosilylation with 23 ppm Pt and 3 phr branched silaneCure Temp Cure Time Elongation Tear Tensile (C.) (min) (%) (ppi) (psi)180 30 941 89 404 190 15 927 97 442 200 10 1249 83 418

EXAMPLE 4

This experiment serves to further demonstrate the effectiveness ofhydrosilylation as practiced in this invention at various cure times. Inthe procedure used polyisoprene (400 grams) was milled smooth as inExample 1. The preferred tetrakis-silane hydride (2 phr) was added nextfollowed by 45 ppm of Pt catalyst. After all the components of thecompounded rubber were mixed on the mill, the flat milled sheet wasconverted into cured tensile sheets by use of a 0.075 inch×6 inch×4 inchmold cavity. The tensile sheets were cured at a variety of moldtemperatures and cure times. See Table 4 where tear strength, tensilestrength and elongation at break as measured with the Instron 5500Rtensile testing machine are shown. TABLE 4 Hydrosilylation with 45 ppmPt and 2 phr branched silane Cure Temp Cure Time Elongation Tear Tensile(C.) (min) (%) (ppi) (psi) 196 5 922 81 368 196 7.5 928 86 395 196 10906 95 434

EXAMPLE 5

This experiment serves to demonstrate the effectiveness ofhydrosilylation as practiced in this invention in the presence of asilica filler and the increased tensile strength that results.Polyisoprene (400 g) was milled smooth as in Example 1. The preferredtetrakis-silane hydride (2.5 phr) was added next followed by 45 ppm ofPt catalyst. Finally, 5 phr of a silica filler was slowly added to therubber while being milled. After all the components of the compoundedrubber were mixed on the mill, the flat milled sheet was converted intocured tensile sheets by use of a 0.075 inch×6 inch×4 inch mold cavity.The tensile sheets were cured at 196° C. See Table 5 where tearstrength, tensile strength and elongation at break as measured with theInstron 5500R tensile testing machine are provided. TABLE 5Hydrosilylation with 45 ppm Pt, 2.5 phr silane, and 5 phr silica fillerCure Temp Cure Time Elongation Tear Tensile (C.) (min) (%) (ppi) (psi)196 12.5 810 114 1728

EXAMPLE 6

This experiment is a comparative study demonstrating that peroxide basedformulations can be optimized for tensile and tear with silica fillersbut elongation remains below 550%. In the procedure used each of therubber samples were initially mixed with 0.02 phr of Ultramarine blueand 0 to 40 phr of amorphous silica to make a non-productive compound. Aproductive compound was subsequently prepared by mixing thenon-productive compound with 1.66 phr of EF(DBDB)-60 peroxide and 2 phrof Sartomer SR350 peroxide co-curing agent. The compounds were thencured and the physical properties of the samples were evaluated. Thetensile strength, elongation at break and tear strength of the curedrubber samples is reported in TABLE 6 Peroxide vulcanization SilicaFiller Elongation Tear Tensile (phr) (%) (ppi) (psi) 0 191 86 163 10 429128 1182 20 476 123 2077 40 524 187 2864

EXAMPLE 7

This experiment is a comparative study demonstrating that sulfur basedgum formulations can be optimized for tensile strength by increasingsulfur content at decreased elongation. For this example, 114 phr ofpolyisoprene was initially mixed with 3 phr of zinc oxide and 2 phr ofstearic acid to make a non-productive compound. A productive compoundwas subsequently prepared by mixing the non-productive compound with 0.5phr MBTS, 0.25 phr MBT, 0.4 phr DOTG/C and 1.0 to 3.0 phr of sulfur. Thecompounds were then cured and the physical properties of the sampleswere evaluated. The tensile strength and elongation at break of thecured rubber samples is reported in Table 7. TABLE 7 Sulfurvulcanization Elongation Tensile Sulfur (phr) (%) (psi) 1 737 1304 2 7051967 3 698 2525

EXAMPLE 8

This example serves to demonstrate the unexpected finding thatpolyisoprene rubber which is clear can be cured into a baby bottlenipple utilizing the curative system of this invention withoutcompromising the clarity of the rubber. Clarity, as a measure of percenttransmittance, was determined using a HACH DR/4000U spectrophotometer.Typical polyisoprene nipples cured traditionally with sulfur or peroxidewere compared to experimental nipples prepared using the hydrosilylationvulcanization technique as described in Examples 1 and 3. As is evidentfrom the comparison in Table 8 nipples prepared following the teachingsof this invention provide cured materials without compromising theclarity of the raw rubber. TABLE 8 Clear baby bottle nipple usinghydrosilylation vulcanization cure. % Nipple Type Transmittance Sulfur3.8 Peroxide 45.8 Pt/Hydrosilylation 78.2 Uncured 85.1 Polyisoprene

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

1. (canceled)
 2. (canceled)
 3. A curable composition as specified inclaim 20 wherein said polyisoprene rubber has a vinyl microstructurecontent of at least 0.5 percent.
 4. A curable composition as specifiedin claim 20 wherein the polyisoprene rubber has a vinyl microstructurecontent within the range of 1 percent to 4 percent.
 5. A curablecomposition as specified in claim 20 wherein the polyisoprene rubber hasa vinyl microstructure content within the range of 3 percent to 8percent.
 6. A curable composition as specified in claim 3 wherein thecrosslinking agent having at least 2 hydrosilyl groups per molecule is atetrakis(dialkylsiloxy)silane.
 7. A curable composition as specified inclaim 3 wherein the crosslinking agent having at least 2 hydrosilylgroups per molecule is tetrakis(dimethylsiloxy)silane.
 8. A curablecomposition as specified in claim 3 wherein the crosslinking agenthaving at least 2 hydrosilyl groups per molecule istris(dialkylsiloxy)methyl silane.
 9. A curable composition as specifiedin claim 3 wherein the crosslinking agent having at least 2 hydrosilylgroups per molecule is tris(dimethylsiloxy)methyl silane.
 10. A curablecomposition as specified in claim 3 wherein the hydrosilylation catalystis present at a level which is within the range of 1 ppmr to 200 ppmr.11. A curable composition as specified in claim 10 wherein thecrosslinking agent is present at a level which is within the range of0.5 phr to 10 phr.
 12. A curable composition as specified in claim 11wherein the hydrosilylation catalyst is present at a level which iswithin the range of 20 ppmr to 100 ppmr.
 13. A curable composition asspecified in claim 12 wherein the crosslinking agent is present at alevel which is within the range of 1 phr to 6 phr.
 14. A curablecomposition as specified in claim 13 wherein the crosslinking agent ispresent at a level which is within the range of 2 phr to 4 phr.
 15. Acurable composition as specified in claim 14 wherein the hydrosilylationcatalyst is present at a level which is within the range of 25 ppmr to75 ppmr.
 16. (canceled)
 17. (canceled)
 18. (canceled)
 19. (canceled) 20.A curable polyisoprene rubber composition comprising: (1) a polyisoprenehomopolymer rubber comprised of repeat units that are derived fromisoprene monomer, (2) a carbonyl inhibited platinum catalyst, and (3) abranched crosslinking agent of the structural formula:

wherein n represents an integer from 1 to about 3, wherein R representsan alkyl group containing from 1 to 4 carbon atoms, a phenyl group, or ahydrosilyl group.